Effects of Different Drying Methods on Untargeted Phenolic Metabolites

molecules Article Effects of Different Drying Methods on Untargeted Phenolic Metabolites, and Antioxidant Activity in Chinese Cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum Dun.) Millicent G. Managa 1, Yasmina Sultanbawa 2 and Dharini Sivakumar 1,* 1 Phytochemical Food Network Research Group, Department of Crop Sciences, Tshwane University of Technology, Pretoria West 0001, South Africa; [email protected] 2 Australian Research Council (ARC), Queensland Alliance for Agriculture and Food Innovation, Center for Food Science and Nutrition, The University of Queensland, QLD 4108 Brisbane, Australia; [email protected] * Correspondence: [email protected]; Tel.: +27-012-382-5303 or +27-012-382-5302 Received: 31 January 2020; Accepted: 9 March 2020; Published: 13 March 2020 Abstract: Chinese cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum are popular traditional leafy vegetables consumed predominantly by rural Africans. Sun drying is adopted as a traditional method of postharvest preservation to store theses leaves during off seasons. The influence of different types of postharvest processing treatments, such as conventional oven drying, solar cabinet drying, sun drying and freeze drying, on the changes on colour properties and antioxidant components were investigated. Freeze-drying retained the ascorbic acid content, antioxidant activities, total chlorophyll content, green colour by reducing the colour difference (DE). With regard to Chinese cabbage and Nightshade leaves, sun and microwave drying respectively had the most negative impact on all the identified phenolic compounds. The OPLS-DA and the UPLC–QTOF/MS and chemometric approach showed kaempferol-3-O-sophoroside, kaempferol-3-sophorotrioside-7-glucoside and hydroxyoctadecenedioic acid as the markers responsible for the separation of sun-dried samples from the other drying treatments in Chinese cabbage. Sinapoyl malate was not detected in sun-dried samples. Caffeoylmalic acid was identified as the marker compound to separate the other drying treatments from the microwave dried samples of Nightshade leaves. Trihydroxyoctadecadiene derivative and hydroxyoctadecanedioic acid were detected in microwaved samples. Due to the cost effectiveness, solar dryer cabinet treatment was recommended for drying both vegetables. The proximate analysis of solar dried functional powder of Chinese cabbage and Nightshade vegetables demonstrated higher contents of protein and dietary fibre. Keywords: Traditional African leafy vegetables; poly phenols; chlorogenic acid; kaempferol glycosides; antioxidants; postharvest processing 1. Introduction Chinese cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum Dun) are popular vegetables among rural and peri-urban households in Southern Africa. The Ca, Fe, 1 1 glucosinolates and β-carotene contents were reported as 1020 g kg− FW, 26 g 36 kg− FW and 1 1 26 g 36 kg− DW respectively, and the kempferol content varied from 0.2002 to 0.25 g kg− on dry weight basis [1,2]. In addition, the raw leaves of Brassica rapa L. subsp. Chinensis contain kaempferol-sophoroside-O-hexoside, kaempferol-dihexoside, kaempferol-sophoroside, kaempferol hexoside, ferulic acid and myrectin-O-arabinoside. Sinigrin was reported as the highest glucosinolate Molecules 2020, 25, 1326; doi:10.3390/molecules25061326 www.mdpi.com/journal/molecules Molecules 2020, 25, 1326 2 of 23 in freshly harvested Chinese cabbage [3]. The Nightshade leaves contained higher levels of 1 1 1 Ca (199 g 100 g− ), Mg (92 g 100 g− ), and Fe (7.2 g 100 g− ) compared to raw spinach [4] and the following phenolic compounds, neochlorogenic, chlorogenic, and caffeoylmalic acid, kaempferol O-rhamnosyl hexoside and rutin [3]. Since, the growing alertness of functional compounds found in the fruits and vegetables and their beneficial health effects in recent years has pointed to the fact that increased inclusion of fruits and vegetables is essential in our daily diets [5]. Traditional African leafy vegetables are well known for the contribution of both micronutrients and functional compounds to the diets of African consumers [6]. Therefore, traditional African leafy vegetables can be included in diet diversification strategy for the sub-Saharan African population to combat the hidden hunger [7]. Generally African leafy vegetables are consumed as soups, or relishes as side dishes with the main meal the carbohydrate staples. Research reports have shown that African vegetables are an abundant source of non-nutritive phytochemicals and their biological activities have been associated with many health benefits, such as anti-diabetic or obesity effects [8]. For example, moringa leaves (Moringa oleifera), rich in flavonoid aglycons quercetin and kaempferol in methanolic extract, showed a significant antidiabetic and antioxidant activity [8]. Leaf extracts of Amaranthus spinosus demonstrated anti-tumour properties opposing liver, breast and colorectal cancer cells [9]. Production of African leafy vegetables in Southern African region is encouraged at home garden or commercial level because they contribute positively towards food production and security due to their draught tolerance [5]. The highly perishable nature of the African leafy vegetables limits their marketability therefore most are dried in shade or under sun as a method adopted for preservation during the off season to remove moisture from the vegetable and to increase shelf life by preventing microbial decay without adding any preservatives [10]. The postharvest drying process and the type of methods adopted can result in changes in functional compounds and physical properties of green leafy vegetables [11]. Furthermore, postharvest drying provides an opportunity to earn a higher income from traditional vegetable functional food. The global functional foods market size is estimated to increase in 2025 to US $275.77, specifically due to the increasing consumer demand for nutritional and fortifying food additives [12]. Amaranthus hybridus, dried at 40 ◦C for 12 h, increased fibre content to 7% and β carotene and vitamin C were decreased at levels of 19.4% and 13.9% respectively [13]. Traditionally adopted sun drying has many drawbacks mainly because it is difficult to control large quantities, to achieve homogenous quality standards and to implement food safety guidelines; also, little is known about the impact of different drying methods on the phenolic compounds, antioxidant activity and ant nutritive compounds in Chinese cabbage and Nightshade leafy vegetables. However, the type of drying method adopted to dry the vegetables for the use of functional ingredients for the development of value-added product must be standardised. Consequently, the objective of this study was to instigate the impact of different drying methods, such as sun drying, shade drying, drying in a solar cabinet dryer, hot air oven and freeze drying methods, on the changes in i) colour properties, ii) changes in lipophilic pigments, iii) phenolic metabolite components, iv) ascorbic acid and v) antioxidant property in Chinese cabbage and Nightshade leafy vegetables. 2. Results and Discussion 2.1. Colour Changes and Lipophilic Pigments Table1A,B illustrate the colour changes that took place during di fferent types of postharvest drying in Chinese cabbage and Nightshade leaves respectively. The following colour coordinates were used to express the colour changes. Light intensity (L*), colour coordinate a* relates to red and green, colour coordinate b* relates to yellow colour, h◦ = arc tangent b*/a* represents a basic colour like red (~29◦), orange (~45◦) or yellow (~70◦), DE-colour difference, the variation in colour compared to the original sample using the following formulae given in Section 3.4. Molecules 2020, 25, 1326 3 of 23 Freeze-dried powers of both types of leaves showed significantly highest L* (luminosity), b*, h◦ values and the lowest a* value, whilst the oven drying showed the opposite effect by showing significantly reduced L*, h◦ values and highest a* value (Table1A,B). On the contrary during hot air (oven) and freeze drying of cabbages (Brassica oleracea L. variety Capitata L.), no differences in brightness (L*) value was reported [14].However, the increase in b* value during postharvest dehydration treatments compared to the fresh leaves observed in Chinese cabbage (Table1A) and Night shade leaves (Table1B) are possibly due to the interference of intercellular air trapped on the greenness of the chlorophyll as explained previously by Rajkumar et al., [14] and confirms their findings. The purchasing power of consumers is determined by the colour (quality) of the products [15]. Thus, the DE value plays a crucial role in relating exact colour changes in dried products; if DE is less than 1.0 it means the colour differences cannot be noticeable [16]. In this study, Chinese cabbage and Nightshade leaves subjected to different drying treatments exceeded the DE value 1.0, and the DE value of the freeze-dried samples showed the lowest (Chinese cabbage, DE = 2.79; Night shade DE = 3.18) values than the samples from other drying treatments. Conversely, the samples from the oven drying method showed the highest DE value (Chinese cabbage, DE = 25.93; Nightshade DE = 14.66). The greater DE values indicated lighter leaves than the fresh and freeze-dried samples. Table 1. Effect of drying treatments on colour changes in Chinese cabbage and Nightshade leaves. (A) Effect of Drying Treatments on Colour Changes in Chinese Cabbage

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